Harvesting heads for agricultural harvesters are being manufactured in greater and greater widths. As the widths increase, additional means have been provided to support the reels for the harvesting heads.
Traditionally, two arms were used to support the reel for a harvesting head, one disposed at each end of the reel. More recently, three arms have been used to support the reel for a harvesting head, one arm at each end of the reel, and one arm in the middle. In the future, if the widths of harvesting heads further increase, four or more arms may be used to support the reel for a harvesting head.
A problem that has surfaced with the three arm reel support arrangement is the inability to keep all of the arms at the same height above the cutting platform of the harvesting head. The spacing of the reels is very important to ensure that the crop is drawn over the reciprocating knife that extends laterally across the front of the harvesting head.
As the agricultural combine with harvesting head attached is driven through the field, the reel is raised and lowered by the hydraulic height control circuit. The operator does this to maintain the optimum height for the crop it is harvesting. As the agricultural combine travels through the field, the operator will regularly change the height of the reel by signaling the hydraulic height control circuit to either increase the reel height (by raising the three arms upon which it is supported) or decrease the reel height (by lowering the three arms upon which it is supported).
Furthermore, as the agricultural combine travels through the field the harvesting head experiences a significant amount of shaking and vibration which tend to bounce the reel as it travels.
This vibration, bouncing, and regular adjustment of the reel will cause the reel support arms to get out of synchronization. Operators preferred that the reel, and hence the reel support arms be at the same height. If one arm “droops”, such as by losing hydraulic fluid in the portion of the hydraulic height control circuit that raises and lowers that arm, it may interfere with the reciprocating knife disposed along the leading edge of the harvesting head.
One solution to this problem has been to provide a hydraulic height control circuit that periodically drives the arms against fixed mechanical stops that hold the arms in the exact same position, and then simultaneously supplements (or bleeds off) the hydraulic fluid in the hydraulic height control circuit for all of the reel support arms. This periodic correction, however, is time-consuming, and requires special modifications to the hydraulic cylinders that are part of the hydraulic height control circuit.
What is needed, therefore, is a hydraulic height control circuit for the reel support arms of a harvesting head of an agricultural combine that will reduce the frequency of calibration of the height of the reel support arms. It is an object to provide such a hydraulic height control circuit, and to provide a harvesting head having such a hydraulic height control circuit.